The double-screw conical mixer, as a mixing device, has been widely used in lots of industries for its high mixing rate and quality. However, a deeper understanding as well as a large-scale simulation on particle mixing in this kind of mixer is still lacking. In this paper, discrete element method (DEM) is utilized to investigate the flow pattern of particles and the mixing performance of the double-screw conical mixer with particle number from 0.5 to 1.1 million. The effects of the rotation speed and the sweeping speed on mixing rate and quality are studied. The particle size ratio is also investigated to reveal the mixing performance of this mixer. The simulation results indicate that particles are well mixed in the overall and local mixing domain as well as in the binary system with a large particle diameter ratio of 1:3. When the rotation ratio is at a specific value, the mixer will have a better mixing performance. Considering both the mixing performance and the power consumption, the optimal condition for operation can be obtained. It also can be found that particle breakage is less likely to occur here. Compared with other parts of the mixer, the wear rate on the screw edge is larger. (C) 2019 Elsevier B.V. All rights reserved.